Real-time signal digitizers and digital oscilloscopes can operate in different modes. In a “free-running”, or a non-triggered mode, a stream of incoming signal samples is stored in memory, or on disk, for subsequent analysis and processing. In a triggered mode, a special trigger signal is required to synchronize acquisition with the trigger signal. The trigger signal can come from external source (external trigger), or can be generated based on specific signal properties. A common example is an “edge trigger”, generated by detecting a signal crossing specified voltage level. Both analog and digital trigger techniques are widely used. Digital triggers have well known advantages, such as reduced jitter level. A real-time digital trigger method is described in U.S. Patent Application Publication No. 2014/0047198. In that application, a digital trigger is generated by a dedicated trigger channel. This channel operates in real time and stores trigger events in a trigger memory. A controller reads out stored data values from a data acquisition memory based on real-time trigger events read from the trigger memory.
U.S. Pat. No. 6,753,677 describes systems for trigger jitter reduction for real time digital oscilloscopes. The triggering methods discussed in that patent are not signal-dependent, but are based on standard methods, such as edge (threshold)-based methods and time delay (hold-off)-based methods. In particular, trigger jitter reduction is based on adjustment of a trigger voltage by analyzing recorded waveforms, followed by an adjustment of the trigger voltage and digital sample interpolation.
The complexity of waveforms is increasing with the advancement of digital communication technologies, high speed transmission links and electronics. Common triggering methods are based on voltage level thresholds and timing delays. These methods are not sufficient for reliable detection and analysis of complicated signal waveforms, such as for example, long pseudo-random sequences distorted by communication channels and noise, or high frequency RF pulses used in radar technology.
The need for advanced signal triggering has been realized since digital oscilloscopes were developed. However, limited waveform memory and digital signal processing capabilities have prevented efficient implementation of complicated signal triggers.
Currently high-end digital oscilloscopes offer advanced triggering options, such as “zone triggering” (see Agilent application note “Keysight Technologies Triggering on Infrequent Anomalies and Complex Signals using Zone Trigger” www.keysight.com), or “Visual triggering” by Tektronix (www.tek.com).These approaches are based on manual selection of specific signal events (e.g., extra pulses or missing pulses inside specified timing intervals), defining well-defined logical sequences of events (e.g., a first timing interval between negative and positive edges exceeding a threshold value, with a next timing interval being smaller than the threshold), and subsequent triggering on detection of those events. However those triggering options are limited to well-defined and simple events, and are not universal.
The prior art describes a number of approaches for waveform-based trigger detection.
U.S. Pat. No. 4,797,936 describes a waveform sequence trigger system based on comparing incoming a digitized signal with a binary sequence generated by an analog comparator set to a specific signal level. A reference signal is converted by a comparator and shift registers to a sequence of logical states, e.g., “1” being assigned to a reference signal being above a threshold and “0” being assigned to a reference signal being below the threshold. Then, an incoming sequence of signal samples is compared to a binary reference sequence and a signal indicative of coincidence generates a trigger event. This method is relatively simple; however it is not flexible enough and based on a dedicated hardware circuit comprising a sequence of analog comparators and shift registers.
U.S. Pat. No. 4,843,309 describes a method and apparatus for correction of timing misalignment between waveforms acquired by digital oscilloscopes using a cross-correlation function. In that patent, repetitive signal waveforms are aligned by calculating a signal cross-correlation function, estimating a waveform timing shift by finding a value of the cross-correlation function exceeding preselected tolerance and aligning the waveforms based on the estimated timing shift. The method of cross-correlation for signal alignment is a standard approach used in signal processing and communication theory. However, direct calculation of cross-correlation requires a significant amount of calculations. For example, real-time alignment of a 500 ns signal waveform acquired with a 20 GS/s sampling rate, requires at least 10 thousand multiplications for each signal sample, which transforms to over 1014 operations per second.
U.S. Pat. No. 6,621,913 describes a digital oscilloscope with trigger qualification based on pattern recognition. A digitized signal is stored in a waveform memory, and a trigger circuit having two comparators, each of which is set to a distinct voltage level. A pattern detection circuit is based on detecting a change of an incoming signal state, e.g., a signal crossing a first level and then a second level, or vice versa, and/or detecting time intervals between such level crossings. When the sequence of detection events coincides with a reference sequence, a trigger signal is generated. This method is more advanced compared to the single-level threshold circuit described in the previously mentioned U.S. Pat. No. 4,797,936; however, it is still limited to a very specific way of generating a trigger signal.
U.S. Patent Publication No. 2014/0108856 describes a system operating in real time which relies on a particular sequence of trigger signals. A trigger sequence checker monitors for a predetermined sequence of triggers, and generates an error signal if that sequence is not found. However, this method is more relevant to a multi-channel triggering configuration, and does not solve the problem of waveform-based trigger generation.
The above cited patents and applications describe different solutions for the signal-based triggering problem. However none of those references combine sufficient flexibility and ease of practical implementations for use in applications of the present disclosure. The signal cross-correlation method described in U.S. Pat. No. 4,843,309 is a well known universal signal detection method. However, that approach requires excessive computational complexity and cannot be easily implemented even using most advanced multi-core parallel digital signal processors. Level-crossing hardware based solutions allow real-time implementation, but require dedicated digital and analog circuits. Those methods are not flexible enough for arbitrary signal detection. For example, the number of shift registers and analog comparators is fixed by a triggering circuit design. Also, and even more importantly, none of the above prior art references the address trigger requirements for the precise timing alignment needed for synchronous signal averaging. Also, the issues of time-domain signal-dependent noise analysis based on signal triggering, are not disclosed in the prior art. Precise time-domain synchronous signal averaging, and noise calculation, determine very special requirements for signal trigger and the concept of reference pattern update which will be addressed below in this disclosure.
The main purpose of this disclosure is (i) to describe a method and apparatus for waveform-based triggering, which is at the same time computationally effective and accurate, and (ii) to describe methods of trigger and pattern updates used for synchronous averaging and time domain noise analysis.